1. Field of the Invention
This invention relates generally to textile composites coated with polytetrafluoroethylene (“PTFE”), and is concerned in particular with a unique method of bonding unsintered and unexpanded PTFE films to flexible PTFE substrates.
2. Description of the Prior Art
For many decades, the industrial fabric marketplace has been using PTFE coated textile composites in a large number of applications. The very large majority of the composites have contained woven fiberglass fabric substrates. Over the years, the coated products have had an acceptable performance in most of the applications, offering thermal, chemical, and mechanical benefits to their users.
The PTFE coated fabrics are used as release sheets in the production of sticky materials and various food products. They can serve as conveyor belts in the same sticky material and food applications. Plastic films are heat sealed into packages using PTFE coated fabrics. Flue gas in power plants is confined to ducts by expansion joints containing PTFE coated fiberglass gas seals. Removable insulation systems contain PTFE coated fabrics to resist the many chemicals that come in contact with the insulation systems in chemical plants.
As with any material, there are limitations to the performance of PTFE coated textiles in industrial service. If the textile reinforcement is fiberglass, the fiberglass can fatigue rather quickly in severe mechanical flexing conditions. High temperatures can thermally degrade the PTFE polymer. Sharp objects can cut the coated composite. A major mechanism of failure that certainly occurs in many applications is the stress cracking of the PTFE coating on the coated product.
As the PTFE coated textile performs in industrial service, it is typically stressed mechanically and thermally. If chemicals are involved, it is stressed chemically as well. The stresses will inevitably take their toll in the form of fissures or cracks in the surface of the coated product. Over time in service, the cracks will broaden, eventually permitting materials to flow through them. The materials eventually come in contact with the product's textile reinforcement. Failure occurs a short time after due to, generally, release problems or reinforcement deterioration.
Because stress cracks in PTFE coated composites often create serious problems in performance, attempts have been made over the years to produce coated products with a more stress crack resistant surface. The best results have been provided by composites containing laminated PTFE films. Because the PTFE films possess mechanical properties that are greater than those found in the coated polymers, the laminated products resist stress cracks, often providing the composite with a longer life in industrial service.
PTFE composites containing laminated PTFE films have been in the marketplace for decades. PTFE films come in three main forms: 1) skived PTFE films; 2) cast PTFE films; and 3) paste extruded PTFE films. All three types are used to produce the laminated products.
Regardless of the type of film being laminated, the process is generally considered to be difficult. To varying degrees, the films are fragile and can be very difficult to handle. The lamination process involves high temperatures that can reach up to 725 F. Finally, pressure or stress must be placed on the films in order to initiate the lamination bond.
Skived PTFE films are produced in a sintered form only. In order to laminate the film, the film must be heated to the melt point of PTFE, which is around 650 F. During the heating process, the skived film will undergo thermal expansion. Wrinkles in the product will develop during the expansion, so the lamination process must be able to minimize the wrinkle formation during lamination. Additionally, the skived product can be difficult to bond using fluoropolymer adhesives. Due to these problems, only limited amounts of PTFE/fiberglass laminated composites incorporating skived PTFE films are found today in the marketplace.
Cast PTFE films are also produced in a sintered form only. While they are easier to seal using fluoropolymer adhesives, they still offer the same challenging thermal expansion problems associated with laminating skived PTFE films. Also, the cast films can only be produced to a thickness in the range of 0.004″ to 0.005″. Fiberglass laminates containing cast PTFE films are readily available in the marketplace and are being used for numerous industrial applications.
Paste extruded PTFE films are extruded in an unsintered unexpanded form at a specific gravity that typically ranges from 1.5 to 1.6. The unsintered and unexpanded PTFE films are easier to laminate for a number of reasons. First of all, because the film is at a specific gravity that is much lower than sintered PTFE, which has a specific gravity ranging from 2.1 to 2.3, the film does not experience the thermal expansion problems seen in cast and skived films as it is being heated to sintering temperatures—around 650 F. As a result, the unsintered film is easier to control during lamination and the finished composite can be more readily produced with fewer wrinkles and defects. Also, the unsintered film, as it is being heated to sintering temperatures, is more inclined than other PTFE films to adhere to sintered PTFE surfaces during lamination. Fluoropolyer adhesives are generally not needed to effect a bond between the unsintered film and the adjacent PTFE surface. Applying elevated temperature and pressure conditions with a reasonable residence time are usually sufficient conditions for creating a laminate. For these reasons, PTFE fiberglass laminated composites containing extruded PTFE films can be found in many industrial applications today around the world.
However, the unsintered PTFE films, by their nature, are very fragile. They can be easily damaged from only the slightest stress. Thus, they must be handled with great care prior to the bonding of the unsintered film to the substrate. The widths of the unsintered films, which were first manufactured decades ago, have remained very narrow until recent time. Extruded unsintered film widths were typically 6″ or 8″ wide. The unsintered films have been traditionally slit into narrows widths, such as ¼″ or ½″, for service in a number of industries including the wire/cable industry.
Over the last decade, a demand has emerged for wider unsintered film widths. As a result, the films can now be readily found in widths of 12″ or even 14″. The wider widths are required because the unsintered films are being laminated onto PTFE coated fiberglass composites for the purpose of creating laminated barrier materials for severe chemical service and other challenging industrial conditions. As the unsintered film width increases, the difficulties associated with handling and laminating the unsintered film also increase. If not accomplished properly, the resultant laminate will contain wrinkles and, in general, a non-uniform surface. The laminate will possess an aesthetically displeasing, defective, appearance that could very likely be porous and, as a result, be ineffective as a barrier to fluids.